1. Technical Field
The present invention relates to a capacitive proximity sensor installed, for example, in a door handle of a vehicle equipped with a keyless entry system or the like, which detects the proximity or contact of a human body (hand or the like), and to a door handle device comprising this capacitive proximity sensor.
2. Related Art
Recently, automobiles are equipped with keyless entry systems, which can lock and unlock doors without key operations. Keyless entry systems comprise: an authentication unit provided within the vehicle; a portable device carried by a user; a proximity sensor, provided on a door handle at the exterior of the vehicle, which detects contact when a user contacts a predetermined position on the door handle; a locking unit, which locks the door; and an unlocking unit, which unlocks the door.
Known proximity sensors include capacitive proximity sensors, which detect the proximity of an object on the basis of changes in the capacitance of a sensor electrode.
Japanese Patent Laid-Open Publication No. 2002-39708 describes a capacitive proximity sensor in which a series resonance circuit is formed, having a resonance frequency fr, in which a resonant capacitance is the capacitance of a resonance circuit, which includes the electrode capacitance of a sensor electrode, and this resonant capacitance and a resonant inductor are connected in series, this series resonance circuit being caused to resonate at an excitation frequency f0 that is higher than the resonance frequency fr, and objects are detected on the basis of the resonance voltage of this series resonance circuit.
With the capacitive proximity sensor described in Japanese Patent Laid-Open Publication No. 2002-39708, by setting the excitation frequency f0 higher than the resonance frequency fr, the resonance voltage will always decrease when an object approaches the sensor electrode, and therefore it is possible to detect the object from the change in the resonance voltage.
Furthermore, Japanese Patent Laid-Open Publication No. 2002-57564 discloses a human body detector, which detects only human bodies, distinguishing between human bodies and raindrops, on the basis of changes in the output frequency, or the output within a predetermined time, when the output of a capacitive sensor is received.
However, in the capacitive proximity sensor described in Japanese Patent Laid-Open Publication No. 2002-39708, only the decrease in the resonance voltage, as compared to the state in which an object is not present in the vicinity of the sensor electrode, is detected, and therefore it is essentially not possible to distinguish between different objects (for example, a human body and water).
Furthermore, the human body detector described in Japanese Patent Laid-Open Publication No. 2002-57564 is premised on the output changing more rapidly when contact is made by water than when contact is made by a human body, but depending on changes in the environment, such as temperature, and depending on the circuit constants of the detection circuit, it can be difficult to distinguish between a human body and water.